To achieve desired transmission rates in digital transmission systems, techniques for improving system reliability are applied, such as increasing the transmittable quantity of information and reducing the error rate in reception by using both multivalue digital modulation techniques and error correction techniques, and reducing the required CNR (Carrier to Noise Power Ratio) by diversity combining techniques using multiple antennas.
For example, U.S. digital terrestrial broadcasting employs multivalue VSB modulation as its modulation system and recovers transmitted data by using, as error correction techniques, Viterbi decoding, which is effective in decoding trellis coded modulated signals, and Reed-Solomon decoding techniques for decoding Reed-Solomon codes (see, for example, Patent Reference 1, Patent Reference 2, and Patent Reference 3).
In general, a Viterbi decoder calculates branch metrics, indicating likelihoods, between the received signal constellation after phase and amplitude correction (also referred to as equalization below) and the uniquely determined signal constellation that depends on the modulation method. All possible surviving paths are found, the branch metrics of the paths are cumulatively summed, and the path with the smallest cumulative sum is selected. The state of the selected path is output as the Viterbi decoding result to recover the transmitted data.
As methods of reducing the error rate of Viterbi decoding results, techniques relating to the method of calculating the branch metrics have been proposed (see, for example, Patent Reference 4 and Patent Reference 5).
Methods for improving reception performance by diversity combining techniques are described in Patent Reference 6 and Non-Patent Reference 1.